Cu(II)-inhibitory effect on photosystem II from higher plants. A picosecond time-resolved fluorescence study.

Abstract

The influence of Cu(II) inhibition on the primary reactions of photosystem II (PSII) electron transport was studied by picosecond time-resolved fluorescence on isolated PSII membranes. The fluorescence decay from Cu(II)-inhibited PSII centers showed a dominant amplitude of a fast phase (100-300 ps) similar to PSII centers in the uninhibited "open state" and minor contributions of components around 600 ps and 2.6 ns. These data indicate efficient primary charge separation in PSII membranes incubated with Cu(II). The quantum yield of primary reactions in the inhibited PSII centers was similar to that of "open" PSII centers. Kinetic analysis of the decay curves in the framework of the exciton/radical pair equilibrium model showed no significant changes in the rate constants associated with the charge separation/recombination equilibrium. However, in closed centers (QA reduced), a decrease in the rate constant K23, associated with the back-reaction of a relaxed radical pair, by a factor of 4 was calculated. The free energy losses upon primary charge separation (delta G1) and during subsequent radical pair relaxation (delta G2) were also determined in Cu(II)-inhibited centers and were compared with uninhibited centers. No changes in the delta G1 values and a significant decrease in the delta G2 values were found as compared with those of control PSII centers in the "closed" state. These data indicate that Cu(II) does not affect primary radical pair formation, but strongly affects the formation of a relaxed radical pair, by neutralizing the negative charge on QA- and eliminating the repulsive interaction between Pheo- and QA- and/or by modifying the general dielectric properties of the protein region, surrounding these cofactors. Moreover, a close attractive interaction between Pheo-, QA-, and Cu2+ can be proposed. Our results are in good agreement with very recent EPR results indicating an additional effect of Cu2+ on the acceptor side [Jegerschöld et al. (1995) Biochemistry 34, 12747-12758].